1. Field of the Invention
The present invention relates to a position detection apparatus for use in controlling position of a movable portion of a machine tool or industrial apparatus and in particular, to a position detection apparatus for detecting a position of a movable portion performing a linear movement.
2. Description of the Prior Art
Conventionally, a position detection apparatus such as a scale apparatus has been used for performing a position control of a movable portion which performs a linear motion.
The scale apparatus generally includes a main body and a head slider provided in the main body in such a manner that the head slider can move in a linear motion.
In the main body, there is provided, a recording medium shaped, for example, in a round bar, and called "scale". On this scale, a position signal is recorded along the longitudinal direction of the scale. In the position signal, pits and marks are repeated a predetermined number of times. For example, the position signal is a magnetic signal in which the polarity is reversed with a predetermined recording wavelength. Moreover, the head slider has a detection head for detecting the position signal recorded on the scale. This detection head is, for example, provided at a position facing to this position signal and moves in the longitudinal direction of the scale together with the linear movement of the head slider.
In such a scale apparatus constituted by the main body and the head slider is mounted on a reference block and a movable block of a machine tool performing a relative linear movement. That is, the main body or the head slider is fixed on the reference block, and the other is fixed on the movable block. Here, the main body is mounted in such a manner that the longitudinal direction of the scale is in parallel to the movement of the movable block. Accordingly, in this scale apparatus, as the movable block moves in a linear motion, the relative position between this main block and the head slider is changed.
In this scale apparatus having the aforementioned configuration, the detection head provided in the slider head detects the position signal changing according to the change of the relative position, so as to control the position (shift amount) of the movable position.
The main body of the scale apparatus having a long scale for detecting a linear movement position of a machine tool or the like is generally mounted on a machine tool using bolts.
FIG. 1 is a cross sectional view of a main body of a conventional scale apparatus mounted on a reference block of a machine tool. Fig. 1A is a cross sectional view of the main body as viewed about a line vertically intersecting the longitudinal direction of a scale provided in the main body. Fig. 1B is a cross sectional view of the main body as viewed about a line parallel to the longitudinal direction of the scale provided in the main body.
The main body 100 of the scale apparatus has a frame 101 and a bar-shaped scale 102 which are fixed by a bracket 103. Moreover, the frame 101 is fixed to a reference block 110 of a machine tool using fixing members such as a bolt 104, a first washer 105, and a second washer 106.
In the frame 101, the first washer 105 is arranged in such a manner that the center of the insert hole for inserting the bolt shaft is matched with the center of the frame mounting hole 107 provided in the frame 101. Moreover, between the frame 101 and the reference block 110, the second washer 106 is arranged in such a manner that the center of the bolt insert hole is matched with the center of the frame mounting hole 107. The bolt 104 has a bolt head 108 contained inside the frame 101 and a bolt 108 extending through insert hole of the first washer 105, the frame mounting hole 107, and the insert hole of the second washer 106, protruding out of the frame 101. A tightening force is given to the bolt 104 in the direction (Z.sub.2 in FIG. 1) from the bolt head 108 toward the bolt shaft 109, so that the frame 101 is fixed to the reference block 110.
The main body 100 fixed by such fixing members is normally is fixed at both ends of the longitudinal direction of the frame 101, considering the reduction in the size of the fixing members and mounting convenience.
Here, the abutment face between the bolt head 108 and the first washer 105, the abutment face between the first washer 105 and the frame 101, the abutment face between the frame 101 and the second washer 106, and the abutment face between the second washer 106 and the reference block 110 are usually not a completely flat plane but there exists fine warps, curves, and bending. Moreover, in general, the bolt shaft 109 is not completely vertical to the bolt head 108 but there exists a fine inclination. Furthermore, because it is difficult to tighten the bolt shaft 109 completely vertical to the longitudinal direction of the scale 102 and accordingly, it is difficult to fix the bolt shaft 109 completely vertical to the longitudinal direction of the scale 102.
For this, in the conventional scale apparatus, in order to increase the fixing force for fixing the main body 100 to the reference block 110, for example, it is not uncommon to increase the diameters of the fixing members including the bolt head 108, the first washer 105, and the second washer 106, and increase the tightening force from the bolt head 108 to the bolt shaft 109. However, in this case, a bending moment M.sub.3 in the longitudinal direction of the scale 102 is applied to the frame 101, resulting in a deformation of the frame 101 and the scale 102, for example, in a bow shape.
Thus, in the conventional scale apparatus, it is difficult to mount the longitudinal direction of the scale 102 in parallel to the movement direction of the movable portion of the machine tool. This often deteriorates the accuracy of the position detection.